Two days later, he and his co-founder John Hinnegan began the TechStars NYC startup accelerator program and got to work on ThinkNear.

ThinkNear wanted to help brick and mortar businesses gain foot traffic during their down times by buying targeted mobile ads for extra, on-the-fly, marketing pushes.

It sounded good in theory, but Portnoy and his team soon learned that there were some flaws in the business. They had trouble buying highly targeted mobile ads in bulk. ThinkNear switched gears and became a mobile, location-based ad network instead. It began working with advertisers to time mobile ads and then use GPS technology to lead users into stores.

For example, ThinkNear could help Starbucks show a mobile ad for coffee at 10 AM to someone who was within 100 meters of a store. Once the ad was clicked, ThinkNear would give turn-by-turn directions to the nearest Starbucks.

ThinkNear signed up a number of big-name advertisers, including Blackberry and Quiznos.

Today, ThinkNear was acquired by Telenav for $22.5 million. ThinkNear is being rolled up into a new Telenav initiative, Scout Advertising. Portnoy will be Scout's Head of Network, and its twelve employees will remain in Los Angeles.

ThinkNear's acquisition is the biggest exit to date in TechStars' entire 6-year history. It's also the first exit for Roger Ehrenberg's IA Ventures.

"During one of the early days of TechStars and after a particularly harsh day, I tweeted: 'Startup are like roller coasters...except there are no seatbelt and a lot of people do end up falling off,'" Portnoy writes on his blog. "We definitely had our share of downs, but we were fortunate to experience many more ups."